[X86] Teach computeKnownBitsForTargetNode about MUL_IMM

This patch teaches computeKnownBitsForTargetNode about MUL_IMM.

MUL_IMM comes up in certain select of constants.  Specifically, it is
used to multiply the result of SETCC.  Computing the known zero bits
of MUL_IMM allows matchAddressRecursively us to convert some OR into
ADD, which eventually becomes a part of LEA.

This patch fixes:

https://github.com/llvm/llvm-project/issues/61365

Differential Revision: https://reviews.llvm.org/D146787